Mark Alert As Read

Mark matches as seen — the ids in the body (the page just rendered), or every match of the alert when there is no body.

Per page, deliberately: with 165 pages, "seen" has to mean the cars that were actually on screen, or one open would retire an alert the reader has barely looked at. Only rows still unread are touched — this used to rewrite every match on every open (39,326 rows, 3.3 s, on the largest alert). read_at is stamped on the server clock, the same clock /cars uses for visit, which is what keeps a car marked during a visit from being sorted to the end before the visit is over.